Geologic formations below the surface of the earth may contain reservoirs of oil and gas, which are retrieved by drilling one or more boreholes into the subsurface of the earth. The boreholes are also used to measure various properties of the boreholes and the surrounding subsurface formations.
Deep transient logging while drilling (LWD), especially “look-ahead” capability, has been shown to have a great potential in formation evaluation and measurement, such as in predicting over-pressed zones, detecting faults in front of a drill bit in horizontal wells and profiling salt structures. These applications typically require depth of investigation (DOI) ranges of about 5-50 meters. One challenge that occurs during deep transient LWD operations is the generation of a parasitic signal due to conductive drill collars or other downhole components.
A variety of techniques have been proposed to reduce this signal in acquired data. Exemplary techniques include using ferrite and copper shielding, using reference signal for calibration purposes and using asymptotic behavior of the conductive collar time response to filter out the collar signal.
In the case of the target DOI being up to 50 meters, the conductive collar signal is typically more than two orders of magnitude greater than the formation signal even if ferrite and copper shields are used. Then the accuracy of bucking and filtering may not be sufficient to facilitate measurements.